1. Field of the Invention
The present invention relates generally to lenses for distributing light from a light source and more particularly to a lens for changing the effective direction of light emission for a light source.
2. Description of the Related Art
The use of LED's in warning and signaling lights is well known. Older models of LED's produced limited quantities of light over a relatively narrow viewing angle centered around an optical axis of the LED. These LED's were typically massed in compact arrays to fill the given illuminated area and provide the necessary light output. Modern LED's produce significantly greater luminous flux per component, permitting fewer LED's to produce the luminous flux required for many warning and signaling applications. It is known to arrange a small number of high-output LED's in a light fixture and provide each high-output LED with an internally reflecting collimating lens. The collimating lens gathers light from the LED into a collimated beam centered on the LED optical axis. Such an arrangement typically does not fill the light fixture, resulting in an undesirable appearance consisting of bright spots arranged against an unlit background. Light-spreading optical features on the outside lens/cover are sometimes employed to improve the appearance of the light fixture.
FIG. 1 illustrates a prior art collimator of a configuration frequently employed in conjunction with LED light sources. Light from an LED positioned in a cavity defined by the collimator is organized into a collimated beam aligned with the optical axis of the LED. The known internally reflecting collimator for an LED is a molded solid of light transmissive plastic such as acrylic. The radial periphery of the collimator is defined by an aspheric internal reflecting surface flaring upwardly and outwardly to a substantially planar light emission surface. The bottom of the collimator includes a cavity centered over the LED optical axis. The cavity is defined by a substantially cylindrical side-wall and an aspheric upper surface. The aspheric upper surface is configured to refract light emitted at small angles relative to the LED optical axis to a direction parallel with the LED optical axis. The shape of the aspheric upper surface is calculated from the refractive properties of the air/acrylic interface, the position of the LED point of light emission relative to the surface and the desired direction of light emission, e.g., parallel to the LED optical axis. The mathematical relationship between the angle of incidence of a light ray to a surface and the angle of the refracted ray to the surface is governed by Snell's Law: “The refracted ray lies in the plane of incidence, and the sine of the angle of refraction bears a constant ratio to the sine of the angle of incidence.” (sin θ/sin θ′=contant, where θ is the angle of incidence and θ′ is the angle of refraction)
To allow the collimator of FIG. 1 to be easily extracted from a mold, the substantially cylindrical side-wall of the cavity is typically canted at an angle of between 1° and 3° relative to the central axis of the collimator. The cavity is narrower at the top where the side-wall joins the aspheric upper surface than at the bottom of the collimator. For any particular point on the substantially cylindrical side-wall, the path of light refracted into the collimator can be calculated using Snell's law. The shape of the peripheral aspheric internal reflecting surface is calculated from the path of light refracted by the substantially cylindrical side-wall surface and the desired direction of light emission, e.g., parallel to the LED optical axis. The resulting aspheric internal reflecting surface redirects light incident upon it in a direction parallel to the optical axis of the LED.
The result is that substantially all of the light emitted from the LED is redirected parallel to the optical axis of the LED to form a collimated beam. This arrangement efficiently gathers light from the LED and redirects that light into a direction of intended light emission. Unless the light is somehow spread, the light from each LED appears to the viewer as a bright spot the size and shape of the collimator.